The purpose of this study is to investigate electrophysiological mechanisms that may be responsible for arrhythmias produced by acute myocardial ischemia. To create ischemia in a blood perfused, in vitro model in which continuous recordings of transmembrane cardiac potentials can be obtained, we will study cardiac allographs transplanted to the hamster cheek pouch. Using standard microelectrode techniques, we will investigate alterations in impulse formation and conduction induced by ischemia, investigate the influence of adrenergic and cholinergic stimulation on these alterations, and attempt to replicate the ischemia-induced changes by manipulating the superfusion media. We will record intracellularly from subepicardial and subendocardial ventricular myocardium, from atrial myocardium and from Purkinje fibers during acute ischemia and compare their responses to similar durations of ischemia. Such data, obtained from cardiac tissue perfused by an intact circulation, rather than superfused, might provide more relevant information on ischemia-induced cellular alterations. This knowledge may lead to better understanding of the pathogenesis of arrhythmias that occur in the intact heart during acute myocardial ischemia.